Lasers have become essential light-sources for uniform illumination in a wide range of applications, including surface inspection of semi-conductor materials, thermal annealing of display-screen glass, and rapid assay of bio-medical fluids. A common requirement is a beam of laser-radiation having an elongated cross-section to uniformly illuminate a line on a surface of a material or a lateral plane in a volume of transparent material. Such beams of laser-radiation are referred to generally as “flat-top beams” or “line-beams”. Other applications require uniform illumination of a rectangular area on a surface of a material.
Optical resonators in many laser-sources have a lowest-order transverse mode that has a cross-sectional intensity distribution described by the Gaussian function. In the absence of optical aberration, optical output from such laser-sources has a Gaussian intensity distribution. Light output from single-mode optical fibers also has a Gaussian intensity distribution, to a good approximation. A beam of laser-radiation maintains a Gaussian intensity distribution, provided any surfaces the beam is reflected from, or any media the beam propagates through do not have optical aberrations.
A beam of laser-radiation propagates along a beam-axis defined by the propagation of the centroid of the cross-sectional intensity distribution. For a beam of laser-radiation having a Gaussian intensity distribution and power “P”, the intensity “I(r)” at a transverse displacement “r” from the beam-axis is:
                                          I            ⁡                          (              r              )                                =                                                    2                ⁢                P                                            πω                2                                      ⁢                          e                                                                    -                    2                                    ⁢                                      r                    2                                                                    ω                  2                                                                    ,                            (        1        )            where “ω” is the beam-radius at
  13.5  ⁢  %  ⁢          ⁢      (          1              e        2              )  of the maximum intensity I(0) on the beam-axis. The beam-radius ω changes as the Gaussian beam of laser-radiation propagates through optical elements and through free space.
Diffractive optical elements (DOEs) provide one means to transform a beam of laser-radiation having a Gaussian intensity distribution into a line-beam. A DOE has a micro-structured pattern of lines or dots that spatially modifies the phase of transmitted laser-radiation. A line-beam is created by mutual interference of rays within the phase-modified transmitted laser-radiation. The size of micro-structured pattern required to create a line-beam by such interference is wavelength specific. For this reason, DOEs have severe chromatic aberration, whereby the width of line-beam created is different for each wavelength within a polychromatic beam of laser-radiation. A polychromatic beam of laser-radiation could have a broad spectral bandwidth or comprise multiple discrete wavelengths. Another disadvantage of DOEs is degraded image quality and power losses, due to rays diffracted into higher orders of the micro-structured pattern.
A Powell lens provides another means to transform a Gaussian beam of laser-radiation into a line-beam. A Powell lens has large spherical aberration in the center thereof, redistributing laser-radiation at the peak of the Gaussian beam away from the beam-axis. Towards the edges thereof, a Powell lens becomes essentially a prism, refracting peripheral laser-radiation of the Gaussian beam at a defined maximum angle from the beam-axis. Overall, transmitted laser-radiation is transformed into a diverging beam having a uniform intensity distribution on both sides of the beam-axis up to the maximum angle.
A Powell lens generally transforms a Gaussian beam of laser-radiation into a line-beam in one plane, as described in U.S. Pat. No. 4,826,299. A rectangular area on a planar surface may be uniformly illuminated by using two orthogonally oriented Powell lenses, located serially in a Gaussian beam of laser-radiation, as described in detail in U.S. Pat. No. 7,400,457. A Powell lens, however, also has severe chromatic aberration, which degrades a line-beam created from a polychromatic beam of laser-radiation.
There is need for an achromatic optical device for shaping a polychromatic beam of laser-radiation having a non-uniform intensity distribution into a beam of laser-radiation having uniform intensity distribution. There is particular need for an achromatic optical device for shaping a polychromatic beam of laser-radiation having a Gaussian intensity distribution into a beam of laser-radiation having a flat-top intensity distribution.